Sound reproducing system



Nov. 15, 1938.

W. B. SNOW SOUND REPRODUC ING SYSTEM Filed Sept. 50, 1936 a2 33x /36 IOFAQDKENCLj 3,1410 conn scnqu eats-Sheet 2 FRONT 40 4V0 CORRECTIONINVENTOR y WB.$NOW

ATTORNEY Ritentecl Nov. '15, 1938 UNITED STATES amen SOUND nnrnonucmoSYSTEM William B. Snow, Maplewood,

N. 3., assignor to I Bell Telephone Laboratories, Incorporated. New

York, N. Y., a corporation oi New York Application September 30, 1936,Serial No. 103,22?

7 Claims.

'This invention relates to the reproduction of soundwith spacialdistribution and the object of the invention is an improved system forproducing the spacial or stereophonic efiect.

5 It is well known in the art that when two microphones spaced in frontof one stage are connected by separate lines to two loud-speakers spacedin front of anotherdistant stage, the

1 sounds reproduced by the loud-speakers appear similar in general tothat of the original sounds actuating the microphones.

Itis also well known that while the localization of the virtual soundsources is quite ac- 15 curate for observers in the central part of theauditorium, it is much-less accurate for observers in lateral positionsparticularly those toward the front of the auditorium. In a copendingapplication of W. B. Snow and J. C. Steinberg, Serial No. 79,882, filedMay .15, 1936, there is disclosed a stereophonic system in which theaccuracy of the localization is improved by arbitrarily ac- 'centuatingthe normal differences in the volume levels of the transmission channelsproduced by the motion of a sound source with respect to the pick-upmicrophones.

Observers in the central part of the auditorium are at substantially thesame distance from both lateral loud-speakers whereas those in lateralpositions are nearer to one of the loud-speakers than to the other sothat sound waves from one loud-speaker reach a given lateral position ina.

loud-speaker.

It was pointed out in the application referred to above that, if withthe real source stationary, an observer moves laterallyfrom a centralposition, the virtual source shifts in the same direc- 40 tion. It willbe observed that this shifting of the which the direct sound reaches theobserver first and applicant has found that it is due, in part at least,to the different arrival times of the direct 4 sounds from the two (ormore) loud-speakers.

In accordance with this invention this undesirable shifting ismaterially reduced by manipulating the "arrival time in the direction ofgiving all observer positions more nearly the same ar- 50 rival timefrom each loud-speaker for sound waves corresponding to original soundwaves arriving simultaneously at both microphones." In general thisrequires that the sound waves projected in a path parallel to the sidewalls of the a'udi 55 torium from each loud-speaker position be detohave a spacial or stereophonic distributionshorter time than sound wavesfrom theother:

virtual source is toward the loud speaker fromlayed with resmct to thosewaves which are projected angularly across the auditorium.

It has been found that the lower frequencies have very little effect onlocalization and that the position of a virtual source is largely deter-5 mined by the higher frequency components of the sound waves. In thepreferred embodiment of the invention, therefore, one low frequency andat least two high frequency loud-speaker units are provided for eachloud-speaker position. The low frequency unit and one high frequencyunit at each position are placed to project sound parallel to the sidewalls and the other high frequency unit is placed with its axis ofprojection turned inward to direct sound waves angularly across theauditorium. The sound waves projected by the 'parallel units are delayedwith respect to those projected by the angular units by electricalnetworks in the circuits feeding these units or by any other suitableform of delay 0 mechanism. The amount of the delay required will varywith the requirements of each particular case and will depend on manyfactors such as the spacing of the units, the size and proportions ofthe auditoriumthe section of the auditorium in. which correction is mostneeded and various others.

It has also been found that this lateral shifting of the virtual sourcewith changes in the observer's position laterally of the auditorium (thereal source being stationary) may also be corrected for any givenportion of the auditorium by arbitrary manipulation of the volume levelsin the several reproducing channels. In other words, as the observermoves laterally to the right, for example, a virtual source which shouldbe localized at center stage shifts toward the right side of the stagebut, if at the same time the volume level of the channel on the leftside of the stage is raised the proper amount, the localized source maybe kept in its proper position. This effect may be utilized to advantagein combina tion with the' manipulation ofthe arrival time as describedmore in detail below.

In the drawings:

Fig. 1 is acircuit diagram of one form of stereophonic reproducingsystem according to the invention;

Fig. 2 is a plan view of the preferred arrangement of the loud-speakerunits; and 50 Fig. 3 is a. diagram illustrating'the application of theinvention to a particular auditorium.

The two-channel system of Fig. l is of the general type described by J.C. Steinberg and W. B. Snow in Electrfial engineering, January 1934, andcomprises essentially two microphones ii, i2 spaced in front of apick-up stage i3 and connected by means of amplifiers M to IT andtransmision lines i8 and i9 to loud-speaker uniis 20, 2| in spacedrelation on a distant reproduviiiv, stage 22. e

The loud-speaker units may be of the general type disclosed in Fig. 5 ofthe article by E. C. Wente and A. L. Thuras in Electrical engineering,January, 1934, and comprise low frequency folded horns 23, 24 and highfrequency horns 25, 26 and 21, 28. The several loud-speakers of eachunit are connected to their respective channels through networks 29, 39which apportion the band of frequencies transmitted between the high andlow frequency speakers in the well-known manner. The network may dividethe band at about 300 cycles in accordance with present practice butfrom the standpoint of proper localization according to this inventionit is quite feasible and in some cases it may be desirable to divide theband at 1,000 cycles or even 2.000 cycles.

For convenience of illustration the high frequency units have been shownalongside the low frequency units but in practice it will usually bepreferable to mount them above the low frequency units as shown in Fig.2 and directed at different angles as described more in detail below.

The amount of delay introduced. the horizontal angle over which the highfrequency units project the sound waves and the distribution of theunits withrespect to the horizontal axis of the reproducing area willvary widely with the requirements of each particular case but thegeneral principles involved will be understood by considering thespecific application illustrated in Fig. 3.

In Fig. 3 the high frequency units are located on the reproducing stagewith a spacing which maybe of the order of twenty feet, with the centersof curvature of the spherical waves emitted from the horns on the line3|. The horns in this case each cover a horizontal angle of 45 .degreesas indicated by the dotted lines 32 to 31, the horns of theloud-speakers 25 and 28 cover the areas between the lines 32, 33 and 36,3! respectively, and the horns of the loud-speakers 26 and 21 cover theareas between lines 33, 34 and 35, 36 respectively.

If we assume that the nearest observers are at adistance at least equalto the spacing of the loud-speakers, the area over which the sound isprojected comprises the two lateral areas 49 and 38, the front centralarea 39 and the rear central area 40. The area 39 receives its directsound without delay from the loud-speakers 2B and 21 and the area 40receives its direct sound from the loud-speakers 25 and 28 which havenetworks 4|, 42 giving equal delays so that the relative arrival timesfor sounds from the two channels is unchanged for these central areas inwhich the localization is quite satisfactory as explained above.

The lateral area 49, however, receives delayed sound from loud-speaker25 and undelayed sound from loud-speaker 21 and since all observerpositions in this area are nearer to loud-speaker 25 than toloud-speaker 28, the delay in the projector from the loud-speaker 25will tend to equalize the arrival times throughout this area. Similarlythe lateralarea 38 receives undelayed sound from loud-speaker 2G andsound from loudaker 28 which is delayed to make the arrival lines morenearly equal throughout this area.

The velocity of sound in air is about 1.100 feet per second so thatabout one mil-second delay is required to equalize the arrival times foreach one-foot difference in the lengths of the air paths from theobserver's position to the two loud-speakers. Since-correction is neededmost in the front lateral positions, the delay introdueed by thenetworks 29 and 30 should ordinarily be chosen to give best results inthese positions. In the present case the arrival times are equalized forpositions 43 and 44 directly in front of the loud-speakers 25 and 28 bynetworks giving delays of about six mil-seconds to correspond with thesix-foot difference between the two air paths in each case. This delaywill also equalize the arrival times for any other positions on thelines 45 and 46. The portions of areas 49 and 38 which are between thelines 45 and 46 will be slightly over-equalized while the portionsoutside these lines will be slightly under-equalized, so that the amountof delay introduced will always be a compromise and will depend upon thelocation of the area in which correction appears to be needed most in aparticular case.

It will be understood that in practice the area covered by eachloud-speakeris not as clearly defined as the drawings would indicate andbecause of this and various other factors, it will generally be foundadvisable to vary the amount of delay introduced, to adjust thepositions of the loud-speakers and make any other changes that seem tobe indicated after the usual listening tests.

The effect of manipulating the arrival time" may be supplemented bymanipulating volume levels in the system shown by ope'raiing theangularly directed loud-speakers 26 and 21 at a higher output level thanthe loud-speakers 25 and 28. Within the correction areas 49 and 38 thehigher level of the sound projected angularly into the area in each casewill supplement the effect of the arrival time manipulation in keepingthe localizing more nearly correct for these lateral positions. Thisarrangement is particularly valuable in cases where sufficientcorrection cannot be obtained by varying the arrival time alone withoutintroducing so much delay as to produce undesirable echo effects.

Since the low frequencies have little effect on localization, asexplained above,'the control of the volume levels may be limited to thehigher frequencies and may be obtained by the use of attenuatingnetworks 41 and 48 in the circuits supplying the loud-speakers 25 and28, by designing these loud-speakers to operate at lower efficiencyparticularly in the upper portion of their range as compared with theloud-speakers 26 and 21, or by any other suitable means.

While the invention has been described with reference to a particularembodiment for purposes of illustration, it will be understood that thegeneral principles of manipulating the arrival time or the volume levelsor both for different observers positions to improve the accuracy oflocalization is equally applicable to stereophonic systems of othertypes. It may be applied to systems having three or more channels and itmay be used in conjunction with other correction schemes such, forexamples, as those using means for accentuating normal level differencesbetween the channels disclosed in the application referred to above. Insome cases it may be advantageous to use three or more directiveloud-speakers at each position with each loud-speaker designed toproject sound in properly timed relation over a smaller or larger anglethan 45 degrees as the case requires.

It will also be understood that the necessary delays may be introducedwithout the use of networks by means of loud-speakers having arelatively long air path between the receiver unit and the horn mouth.Moreover, the several loudspeakers at each position may be replaced witha single unit having a horn with subdividing partitions giving hornsections of different lengths to provide the necessary delays and, ifdesired, of difierent cross-sectional areas'to provide the volume leveldifferences required to assist in correcting localization. The inventionis therefore intended to be limited only by the scope of the followingclaims.

What is claimed is: i

1. In a stereophonic sound reproducing system, two transmission linescarrying currents representing waves from the same sound source atdifferent pick-up positions, and means for reproducing the sounds withspacial distribution comprising loud-speakers spaced in groups before asound reproducing area, each group being connected to one of the lines,two loud speakers, one in each group, disposed with their axes ofprojection substantially parallel to each other, two other,

loud-speakers, one in each group, angularly disposed with their axes ofprojection intersecting each other, and means for delaying theprojection of sound from the parallel loud-speakers with respect to thesound projected from the angularly disposed loud-speakers.

2. A system according to the preceding claim in which sound energy isprojected at a higher volume levelfrom the angularly disposedloudspeakers than from the parallel loud-speakers to improve theaccuracy of localization for lateral observer positions in thereproducing area.

3. In a stereophonic reproducing system, a plurality of transmissionlines carrying currents representing sound waves from the same source atdifferent pick-up positions, a plurality of groups of loud-speakers eachconnected to one of the lines and disposed in spaced relation withrespect to the other groups before a sound reproducing area,'theloud-speakers of each group being disposed to project sound energy todifferent lateral portions of said area, and means for delaying theprojection of energy by some of the loud-speakers to produce more nearlyequal arrival times at all positions within the area for sound wavesprojected to said positions by the several loudspeaker groups.

4. A system according to claim 3 having means for attenuating thedelayed energy projected by some of the loud-speakers to increase theaccuracy of localization in portions of the reproducing area.

5. In a stereophonic sound reproducing system, a plurality oftransmission lines carrying currents representing sound waves from thesame source at different pick-up positions, a sound reproducing area, agroup of loud-speakers connected to each line, said group being disposedin spaced relation before said area, a loud-speaker in each groupprojecting sound waves angularly across the area, and a secondloud-speaker in each group projecting sound waves toward the back of thearea in delayed relationship to the waves projected across the area.

6. A system according to claim 5 having means for supplying energy tothe angularly projecting loud-speakers at a higher level than the energysupplied to the other loud-speakers. r

7. In a stereophonic sound reproducing system, two transmission linescarrying currents representing waves from the same sound source atdifferent pick-up positions, two spaced loud-speakers connected to thelines and projecting sound toward a reproducing area, two other spacedloudspeakers each connected to one of the lines, with their axes ofprojection directed angularly across the reproducing area, and means fordelaying the projection I of sound from the first-mentionedloud-speakers with respect to that projected by the angularly directedloud speakers to improve the accuracy of localization for lateralobserver positions in the area.

' WILLIAM B. SNOW.

